moment vs split second

moment and split second both are nouns.

Word	Noun	Adjective	Verb	Adverb
moment	Yes	No	No	No
split second	Yes	No	No	No

As nouns, split second is a hyponym of moment; that is, split second is a word with a more specific, narrower meaning than moment:

moment: an indefinitely short time
split second: a very short time (as the time it takes the eye to blink or the heart to beat)

Other hyponyms of moment include blink of an eye, flash, heartbeat, instant, jiffy, New York minute, trice, twinkling, wink.

moment (noun)	split second (noun)
having important effects or influence	a very short time (as the time it takes the eye to blink or the heart to beat)
the n-th moment of a distribution is the expected value of the n-th power of the deviations from a fixed value
a turning force produced by an object acting at a distance (or a measure of that force)
a particular point in time
an indefinitely short time
at this time